The present invention relates to intrinsically safe circuits and systems, in general, and more particularly to a multiplexing method and system for a plurality of sensors disposed in a tank containing a potentially explosive environment, and a multiplexer switch for use therein.
In systems which supply signals to and receive signals from a contained environment comprising combustible or explosive vapors, restrictions are imposed to limit the energy over the signal lines penetrating the container below levels that could initiate an ignition of such vapors. For example, in a fuel quantity measurement system, sensors are disposed within a fuel tank and when excited, provide response signals indicative of the fuel level in the tank. Accordingly, each sensor has an excitation signal line and response signal line that penetrate the fuel tank. In order to be intrinsically safe, the circuitry that provides the excitation signals and receives the response signals are specially designed to limit the energy over the signal lines penetrating the tank to within safe levels under normal operating and failure conditions.
Having dedicated intrinsically safe excitation and signal conditioning circuits for each sensor of the contained environment is very expensive and complex. Accordingly, system designers have proposed a multiplexing system in which a group of sensors may be excited from a common excitation circuit and the resulting response signals from the sensors received by a common signal conditioning circuit. Thus, the costs and complexity can be reduced by a factor dependent upon the number of sensors in the multiplexed group. Multiplexing systems conventionally include controlled switches in series with the signal lines.
These multiplexer switches pose certain conditions to the designer of an intrinsically safe system which must be considered. For example, any switch added to the system needs to preserve the intrinsically safe nature of the common excitation and signal conditioning circuits and associated signal lines. However, to operate conventional multiplexer switches, a voltage is generally applied directly to the switch from a power source. Thus, if a failure occurs within the switch, this drive voltage may become directly connected to the corresponding signal line, resulting in a defeat of the intrinsically safe design. Moreover, it would be beneficial to render this preservation of intrinsic safety by maintaining the circuitry of each multiplexer switch to a minimum.
Another consideration results from the line capacitance of the signal lines from the excitation circuit to the sensors and from the sensors to the signal conditioning circuit. This line capacitance acts as a load to ground via a cover shield, for example, and may draw a significant amount of current from the excitation signal. For example, at ten volts AC with a frequency of around eighteen thousand hertz, as much as eleven or twelve milliamps may be drawn through the line capacitance. Conventional multiplexer switches have significant “on” resistance such that the current drawn by the signal line capacitance cause an undesirable voltage drop across the switch, resulting in a measurement error which may be in the range of five percent or so.
Further, reactive loading of the excitation signal can often cause an amplifier driving the signal to become unstable. To enhance stability, circuitry is generally added to the amplifier design. However, the addition of significant switch resistance in series with the excitation signal may interfere with the sensitive stability design of the drive amplifier.
Yet another consideration results from the stray capacitance in the signal line from the sensor back to the signal conditioning circuit. As noted above, a conventional multiplexer switch adds significant in series “on” resistance to the response signal. The current of the response signal which flows through this switch “on” resistance results in a voltage drop across the stray line capacitance, that, in turn, diverts some of the sensor response current to ground through the line capacitance. This undesirable diversion of current may also result in a measurement error.
The present invention ensures preservation of an intrinsically safe design and overcomes the drawbacks of conventional multiplexer switches especially in regard to circuit stability, line capacitance and measurement error noted above.